A condenser microphone includes a conversion element that converts vibrational energy to electrical energy. Such a conversion element may include an electret, i.e., a member capable of holding an electrical charge, which may be made possible by, for instance, applying a voltage to a dielectric material. In the following description, an element that includes an electret will be referred to as an electret element.
An electret is manufactured by, for instance, injecting a charge into an insulating film through corona discharge or adopting the method disclosed in PTL1.
The amount of electrical charge held in an electret decreases over time. This phenomenon is commonly known as charge decay. In addition, a property thereof that does not allow charge to readily decay is referred to as charge stability. The primary cause of charge decay is contact between the electret and water (moisture) in the air.
PTL2 discloses an invention whereby the charge stability in an electret structure is improved by giving it a water-repellent treatment through a hexamethyldisilazane surface treatment (HMDS treatment). However, the charge stability achieved through this invention by forming an extremely thin organic film at the surface of the electret structure is problematic.
Adoption of MEMS (micro electro-mechanical systems) technology has made it possible to form comb-tooth electrodes with a three-dimensional structure and thus manufacture compact conversion elements with greater capacitance between electrodes facing opposite each other. For instance, the conversion element disclosed in PTL 3 includes three-dimensional comb-tooth electrodes facing opposite each other with an electret disposed near a comb-tooth electrode and used as a means for applying a voltage between the electrodes.
A conversion element having three-dimensional comb-tooth electrodes with an electret disposed as described above is bound to have a high aspect ratio, i.e., the ratio of the distance between the electrodes facing opposite each other and the thickness of the comb teeth. There is an issue that needs to be addressed in such a conversion element with a high aspect ratio in that the full effect of HMDS treatment cannot be realized and thus good charge stability cannot be achieved.